Phased array antenna with rainfall drainage channels

ABSTRACT

A PHASE ARRAY ANTENNA BEING PROTECTED AGAINST RAINFALL, COMPRISING AN ANTENNA SUPPORT STRUCTURE, A PLURALITY OF RADIATING ELEMENTS EACH BEING RAISED AWAY FROM SAID SUPPORT STRUCTURE A SUFFICIENT DISTANCE TO PERMIT THE MAXIMUM PROJECTED RAINFALL TO FLOW THROUGH THE CHANNELS   FORMED BY RAISING THE ELEMENTS, AND, MEANS FOR CONNECTING THE ARRAY ELEMENTS TOGETHER AT THE FRONT EDGES TO SIMULATE THE EXISTANCE OF A CONTINUOUS GROUND PLANE.

F6). 16, 1971 FR IZER, JR 3,564,552

PHASE-D ARRAY ANTENNA WITH RAINFALL DRAINAGE CHANNELS Filed June 28,1968 5 Sheets-Sheet 1 INVE/VTUR GEORGE A. FRA/ZER,JR.

ATTORNEY 1971 G. A. FRAIZER, JR 3,564,552

PHASEDARRAY ANTENNA WITH RAINFALL DRAINAGE CHANNELS Filed June 28, 19685 Sheets- Sheet 2 INVENTOR GEORGE A. FRA/ZER, JR

ATTORNEY" 5 Sheets-Sheet 5 INVENTOI? GEORGE A. FRA/ZE/BJR.

ATTORNEY Feb. 16, 1971 G. A. FRAIZER, JR

PHASED ARRAY ANTENNA WITH RAINFALL DRAINAGE CHANNELS Filed June 2 1968=2 5 1.7 A v/vV/////y f x Y I FIG. 36

Feb; 16, 1971 G. A. FRAIZER, JR 3,564,552

PHASED ARRAY ANTENNA WITH RAINF'ALL DRAINAGE CHANNELS 5 Sheets-Sheet 4Filed June 28, 1968 F/G. 4a

FIG. 40

INVENTOR GEORGE A. FRA/ZER, JR.

Feb. 16,1971

6. A. FRAIZER, JR 3,564,552

PHASED ARRAY ANTENNA WITH RAINFALL DRAINAGE CHANNELS Filed June 28, 19685 Sheets-Sheet 5 96 1 9a 14/ v i I; (p SHIFTER /04 as 2 ,WD

DIVERGENT BEAM PROGRAMMER S COLLIMATED a STEERED BEAMS I02 HNVENTORGEORGE A. F/M/ZER, JR.

BYWZAWM ATTORNEY United States Patent O 3,564,552 PHASED ARRAY ANTENNAWITH RAINFALL DRAINAGE CHANNELS George A. Fraizer, Jr., Westford, Mass.,assignor to Ray theon Company, Lexington, Mass., a corporation ofDelaware Filed June 28, 1968, Ser. No. 740,993 Int. Cl. H01q 1/48, 13/00U.S. Cl. 343778 12 Claims ABSTRACT OF THE DISCLOSURE A phased arrayantenna being protected against rainfall, comprising an antenna supportstructure; a plurality of radiating elements each being raised away fromsaid support structure a sufficient distance to permit the maximumprojected rainfall to flow through the channels formed by raising theelements; and, means for connecting the array elements together at thefront edges to simulate the existence of a continuous ground plane.

The invention herein described was made in the course of and under acontract or subcontract thereunder with the Department of the Army.

BACKGROUND OF THE INVENTION This invention relates to antennas having aplurality of radiating elements, and more particularly, to the mountingof the radiating elements in a phased array antenna.

An antenna array is composed of a plurality of radiating elementspositioned in spaced-apart relationship. An example of such an antennaarray is an S-band optically fed phased-array antenna with the arrayface being in the form of a circle of 14-foot diameter and containingfive thousand radiating elements. Each of the radiating elements isconstructed in the form of an open-ended cavity which is terminated andtuned by a flat beryllia window and contains a coupling loop of wire,commonly referred to as a mode launcher, to transmit the microwavesignal through said window. The input signal to each radiating elementis obtained from a microwave phase shifter which is connected to thecoupling loop and imparts the correct phase to the signal transmitted byeach radiating element. Each phase shifter is provided with an inputhorn which intercepts a portion of the electromagnetic radiation of aprimary transmitting horn.

The radiating elements are positioned in spaced-apart relationship bythe antenna support structure which generally is in the form of a heavymounting plate having apertures for receiving each element. With thistype of mounting, the fiat beryllia windows for receiving each elementare flush with the front surface of the antenna support structure so asto provide a flat face for the array.

In this configuration the antenna support structure also serves as aground plane for the radiating elements.

In a typical installation, the antenna array forms a part of a wall orroof of the building which houses the transmitter and peripheralequipment. The face of the array is inclined at an angle ofapproximately 51.5 with the horizontal, as this orientation permits thebeam of radiation to be scanned in elevation from low to high elevationangles.

An important advantage of this type of construction in the antenna arrayis its extreme rigidity and shock resistance. It is far more rigid andshock resistant than the typical radome, as is readily apparent from anexamination of the radome support structure which usually comprises aseries of widely spaced, relatively thin support members in comparisonwith the relatively heavy mounting structure of the antenna array. Thisantenna array is 3,564,552 Patented Feb. 16, 1971 also blast resistantso that it may be located in the vicinity of a missile launch site.

A problem arises when the antenna array is operated in a rainenvironment. The large fiat inclined face of the array intercepts asubstantial quantity of rainfall. As the rainwater flows down the arrayface it combines with the raindrops falling on the lower portions of theface and thereby builds up a sheet of water whose depth increases withdistance from the upper edge of the array face. The depth of the watersheet depends. on the severity of the rainstorm and on the angle ofinclination of the array face as well as the physical size of the arrayface. Since the dielectric constant of water is an order of magnitudegreater than the metallized beryllia glass used in beryllia windows, athin sheet of water in contact with the beryllia windows detunes theradiating elements so that they no longer radiate eifectively. The waterdepth near the lower edge of the array face during a moderate-to-heavyrainstorm is sufiicient to present substantially a dead short to theradiant energy of the radiating elements near the lower edge of thearray.

The rainwater problem exists independently of the type of feed structuresupplying the input signals to the radiating elements so that theproblem is also present with array antennas other than the opticallyfed, phased-array antenna. The windows of the elements near the loweredge of the array are covered with the sheet of water whether they arefiat or in the form of a convex or concave lens, or whether they areconstructed of beryllia or some other material. And furthermore, therainwater problem is also present for array faces having a surface otherthan a fiat plain surface, that is, a curved surface, and in particular,a cylindrical surface.

Accordingly, it is an object of this invention to improve theperformance of an array antenna during pre- In accordance with theinvention an antenna array is provided comprising a plurality ofradiating elements, a support structure which supports and positions theradiating elements in spaced relationship, said support structure beingpositioned behind the array face to form channels which act as adrainage region between the radiating elements to receive such fluid asmay from time to time contact the array face.

An electrically conducting structure interconnecting said radiatingelements preferably is positioned in or adjacent to the array face andforms a ground plane for the radiating elements and is in contact withonly a portion of each radiating element so as to form openings whichare the entrance to the drainage region.

It is preferable that the structure forming the ground plane and thesupporting structure be spaced apart a sufficient distance to presentwith the outer surface of the radiating elements a cavity which servesas a drainage region and with the openings formed by the interconnecting structure provides that the radiation characteristics of the antennaarray remain substantially constant even with a change in the depth ofthe Water in the drainage region.

It should be understood that in the present embodiment each of saidradiating elements have the form of a cylindrical open-ended cavityterminated with a window or lens. However, solid or dipole-type elementsof either metal or dielectric or a combination thereof can be used. Alsoan exit port in one embodiment is provided from which said fluid canflow out of the drainage region.

3 BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of thisinvention will become apparent from the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 shows an enlarged pictorial view of a portion of an array ofradiating elements in accordance with the present invention;

FIG. 2 shows a vertical sectional view of the face of an array alongwith metallic spacers for connection with adjacent elements and exposingthe array drainage channels for disposing collected fluid;

FIG. 3a is a diagrammatic representation of an antenna without a fluiddrainage structure;

FIGS. 3b and 3c show'diagrammatic representations of the antenna with adrainage structure;

FIG. 4a shows an equivalent circuit representation for the impedancepresented to a single radiating element in the case of a flush mountingof the radiating elements in which there are no drainage channels, andno apertures in the ground plane;

FIG. 4b shows an equivalent circuit representation for the impedancepresented to a single radiating element for the situation in which aportion of the ground plane is recessed a small fraction of awavelength;

FIG. 4c shows an equivalent circuit representation for the situation inwhich there is a connecting means bridging a portion of the spacesbetween the radiating elements so that apertures or perforations areformed in the ground plane;

FIG. 4d shows an equivalent circuit representation for the situation inwhich there is the combination of recessed connecting means, aperturesin the ground plane, and enclosed drainage channels;

FIG. 5 shows in diagrammatic form a typical installation of the arrayantenna; and

FIG. 6 shows in diagrammatic form a lens-type, optically fedphased-array antenna incorporating features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there isshown a portion 10 of an antenna array 12, as shown in FIGS. 5 and 6,having radiating elements 14 disposed in spaced-apart relation. Each ofthese radiating elements is supported and positioned by means of anantenna support structure 16 in the form of an electrically conductingplate having apertures to receive the radiating elements 14, saidantenna support structure being located behind the face of the array.Each radiating element 14 is aflixed to the support structure 16 bymeans of mounting lugs 18 and bolts 20 which secure the mounting lugs 18to the support structure 16. Referring to FIGS. 1 and 2, there are shownspring-loaded, metallic, electrically conducting spacers in the form ofshoes 22 which are attached to the radiating elements 14. Each shoe 22is positioned with the aid of a spring 24 and guide pin 26. In thisembodiment, the radiating element 14 has a metallic cylindrical wall ofsufficient depth to support the guide pins 26 which are mounted in saidwall such that the inner end of a guide pin 26 is flush with the innersurface of said wall. The front or transmitting end of each radiatingelement 14 has a metallized beryllia ceramic window 28 brazed to theinner surface of the cylindrical wall of the radiating element 14, suchthat said beryllia window is flush with the front surface of saidradiating element. Integral with and protruding radially from thecylindrical wall of each radiating element 14 is a lip 30 which is flushwith the front surface of said radiating element to facilitate thedischarge of fluid from the front surface of the beryllia window 28. Asis shown in FIG. 1, drainage apertures 32 are provided in the face ofthe array to drain the fluid from the front faces of the radiatingelements 14, each of said apertures having a boundary comprised of thelips 30 of three adjacent radiating elements 14 and the correspondingthree pairs of shoes 22. The shoes 22 are located below the lips 30 toform a recession in the face of the array, and thereby facilitate theentry of fluid from the front surfaces of the radiating elements 14 intothe drainage apertures 32. An electrical ground plane is formed for thearray of radiating elements 14 by the combination of lips 30 and shoes22 lying in or adjacent to the face of the array, said ground planebeing perforated by the drainage apertures 32.

FIG. 2 is a cross-sectional view taken through three radiating elements14, as indicated in FIG.1, and illustrates in detail the spring loadingof a shoe 22 and the interior of a radiating element 14. The springs 24are fitted into recessions in the outer surfaces of the cylindricalwalls of the radiating elements 14 and maintain the necessary pressureto urge the shoes 22 radially outward from the cylindrical walls of saidradiating elements so that contact is made between pairs of shoes 22 ofadjacent radiating elements 14 with sufficient pressure to provide goodelectrical conduction. Each radiating element 14 has a base 34 whichcloses off one end of the cylinder formed by the walls of said elementto form an open-ended cavity 36 which in combination with the couplingloop 38, mode suppressing rod 40, and beryllia window 28 comprise themicrowave structure for transmitting microwave radiation. The microwaveenergy enters the radiating element 14 by means of a coaxial connection,not shown, and the coupling loop 38. Provision is made for the coaxialconnection by a circular opening 42 in the base 34 of the radiatingelement 14 which opening has a diameter larger than the diameter of thewire of loop 38 such that said wire can pass through said opening withthe necessary clearance between said wire and the boundary of saidopening, said clearance being similar to the clearance between the innerand outer conductors of said coaxial con nector, not shown, so that themicrowave energy can be readily coupled through said coaxial connectorinto said radiating element. Coupling loop 38 is constructed of a rigidlarge diameter wire bent with two ninety degree bends to form twoparallel arms one of which connects with the aforementioned coaxialconnector and the second arm being affixed by means of screw 44 to thebase 34. The diameter of the wire of coupling loop 38 is sulficientlylarge to present an adequate radiating surface on the wire and, as iswell known, to effect the necessary capacitance to the wire. Cavity 36also contains a wellknown, mode-suppressing rod 40 which is affixed, bybrazing, to the center of the beryllia window 28. The diameter of themode-suppressing rod 40 is approximately twothirds the diameter of thewire of the coupling loop 38, the length of the mode-suppressing rod 40is approximately twenty percent greater than the overall length of thetop portion of the coupling loop 38 where said top portion is theportion of the coupling loop 38 which is parallel to the beryllia window28. The two ends of the mode-suppressing rod 40 are bent away slightly,approximately twelve degrees, from the surface of the beryllia window28, and said tuning rod is so mounted that the plane containing thecenter line of said rod is perpendicular to the plane containing thecenter line of the coupling loop 38. The overall diameter of saidradiating element is approximately two inches at S band.

In operation, therefore, the drainage apertures 32, as shown in FIG. 1,admit fluid such as rainwater which from time to time contacts theberyllia windows 28, the lips 30 and the shoes 22 of the radiatingelements 14. The fluid then flows upon the support structure 16 andbetween the radiating elements 14 in enclosed drainage channels 46 andthereby drains away from the face of the array. The enclosed drainagechannels 46 form a microwavecavity-type structure which in combinationwith the drainage apertures 32 inhibit the formation of undesirablesurface waves on the face of the array and is substantially free ofresonances which would significantly disturb the performance of theantenna array 12.

FIGS. 3a, 3b, and 3c illustrate, with the aid of diagrammaticrepresentations of antenna arrays, the operation of this embodiment ofthe invention in the removal of fluid from the face of the array and theresulting improvement obtained therefrom. FIG. 3a shows a flush mountedantenna array of the prior art in which the radiating element 54 and thesupport structure 56 support a sheet of water 48 of the face of thearray. The sheet of water 48 alters the impedance presented to theradiating elements of the prior art and thereby detunes the elements.The sheet of water 48 does not appear on the face of the array in thepresent embodiment of FIG. 30 because the water has been withdrawn fromthe face of the array by means of the enclosed drainage channels 50. Anintermediate solution shown in FIG. 3b utilizes open drainage channels52.

Referring specifically to FIG. 3b, the open drainage channels 52 can beformed by raising the radiating elements 54 away from the supportstructure 56 of FIG. 3a, or equivalently, by removing material from saidsupport structure between said radiating elements. The resultant supportstructure 58 is shown in both FIGS. 3b and 30. In FIG. 3b, the waterdrains into the open drainage channels 52 and does not build up into asheet of water 48 to cover the radiating elements 54. In someapplications where this intermediate solution can be used, the supportstructure 58 with the water 48 aid in forming the ground plane. However,for optimum operation of the antenna array, the radiating elements 54are tuned to resonate with the impedance presented by the drainagechannels. The effective depth of the open drainage channels 52, andhence their impedance, varies with the quantity of the water in thechannel. This variation of impedance with water depth in the drainagechannel is virtually eliminated, as shown in FIG. 30, by connecting thefront edges of the raised radiating elements with shoes 60, therebyforming the enclosed drainage channel 50. In this configuration, theimpedance presented to the radiating elements 54 remains essentiallyconstant with changing water depth so that the radiating elements 54 canbe tuned, and remain tuned whether or not the water is present.

The operation of the enclosed drainage channels 50 can be furtherexplained in terms of the electrical resonances introduced by thevarious components of the embodiment shown in FIG. 1. One possibleexplanation for the effects on the array impedance which are introducedby the perforated and recessed portions of the ground plane is herebypresented. These effects are directly related to the dimensions of theperforated and recessed portions of the ground plane and to the depth ofthe enclosed drainage channels 46. The depth of the enclosed drainagechannels 46 are less than a half wavelength, to avoid resonances withinthe band of interest. For optimum performance, the depth isapproximately a quarter wave length. Six equally spaced areas of contactaround the circumference of each element are provided by the location ofthe shoes to permit proper current flow between the elements and toavoid ring resonances.

The effects on the array impedance which are introduced by theperforated and recessed portions of the ground plane can be determinedwith the aid of a representation of the enclosed drainage channels, theperforations or apertures in the ground plane, and the recessedconnecting means in the form of an equivalent circuit in which thecircuit parameters are related to the physical structure of the singleelement of FIG. 1 with the help of the unit cell 62. For purposes ofanalysis, unit cell 62 is taken as the region enclosed by a rightcylinder coaxial with the radiating element 14 and extending from thesupport structure to the face of the array, and having a cross sectionin the form of a hexagon of which each side is the perpendicularbisector of a line segment extending from the center of said radiatingelement to the center of an immediately adjacent radiating element 14.The equivalent circuit is developed with the aid of FIGS.

4a, 4b, 4c, and 4d. The circuit impedances are normalized to theintrinsic impedance of free space. The impedance, normalized, of aradiating elment. 14 at the face of the array is taken as unity.

Referring specifically to FIG. 4a, there is shown a simplified drawingof a portion of the face of the antenna array which for illustrativepurposes has been modified so that there are no recessed and noperforated portions in the ground plane and no drainage channels, saidportion comprising one unit cell 62. The area 64 within the unit cell 62and Without the periphery of the beryllia window 2-8 is electricallyconducting and functions as the ground plane. The impedance presented tothe radiating element 14 is given by S, the normalized characteristicimpedance of free space, where the terminal pair 66 represents theconnection to the radiating element 14. The expression for the impedanceS includes the effects of scan angle 0, thus, S=1/ cos 0 for H planescan and S=cos 0 for E plane scan. The equivalent circuit, therefore,con sists of simply a resistor 68 of value S.

Referring specifically to FIG. 4b, there is shown a portion of the faceof the antenna array, said portion being the same portion as is shown inFIG. 4a. However, in FIG. 411, this portion of the array has beenmodified to i show a recession in the ground plane, and no drainageapertures and no drainage channels are shown. The area 70 indicates themetallic electrically conducting region forming the recessed connectingmeans. The area 70 is combination with the area of the element lip 30equals the area 64 shown in FIG. 4a. The recession of the electricallyconducting region 70 introduces an additional impedance 72 which has theform of a shorted line where the length of the line is represented by A,the depth of the recess. In the electrical schematic representation, theimpedance presented at the terminal pair 66 is therefore the combinationof the resistor 68 in series with the recession impedance 72.

Referring specifically to FIG. 4c, there is shown a portion of theantenna array, said portion being the same portion shown in FIG. 4a.However, in FIG. 40, this portion has been modified to show the portions74 of the drainage apertures 32 within the unit cell 62, with norecession in the ground plane and no drainage channels being shown. Theperforation of the ground plane introduces an additional component tothe impedance presented to a radiating elecent 14 at the terminal pair66. The additional element is a reactance 76 of value X where X dependson the dimensions of the drainage apertures 32. X is found byapproximating the drainage apertures 32 with circles having the sameareas as said apertures. X is given by 41rd sAn where A is the area ofone unit cell 62 and x is the wavelength of the transmitted signal. Thediameter d of the equivalent circle 78 is given by where A is the sum ofthe areas of the six portions 74 of the six drainage apertures 32 lyingwithin the unit cell .62, and the factor of two in the denominatorarises from the fact that A also equal the sum of the areas of twodrainage apertures 32, and that each portion 74 has an area equal toone-third the area of a drainage aperture 32. The equivalent circle 78and its radius r is shown in FIG. 40. Thus, the impedance presented to aradiating element 14 at terminal pair 66 is the series combination ofresistor 68 of value S and the aperture reactance 76 of value XReferring specifically to FIG. 4d, there is shown the electricalequivalent circuit of a portion of the face of the antenna array, saidportion comprising one unit cell 62. In this representation said portionincludes the recessed and perforated portions of the ground plane, andthe enclosed drainage channels 46 shown in FIG. 1. The impedance 80presented by the enclosed drainage channels 46 is convenientlyrepresented by a shorted line where the length of the line y is relatedto the depth of the enclosed drainage channels 46. As is shown in theelectrical schematic of FIG. 6d, the channel impedance 80 appears inparallel with the aperture reactance 76 of value X The optimum value forthe length of the shorted line of the channel impedance 80 is indicatedin FIG. 4d. When the fluid in the enclosed drainage channel 46 is ofsuch a depth that the equivalent length of the shorted line of thechannel impedance 80 is a quarter wavelength, the channel impedance 80is essentially infinite so that the combination of aperture reactance 76and channel impedance 80 is essentially equal to the value X In theelectrical schematic representation the aperture reactance 76 appears atthe end of the transmission line representation of the recessionimpedance 72. The combination of aperture reactance 76 and the recessionimpedance 72 is the value X reflected to the beginning of the linerepresenting the recession impedance 72, and similarly, the combinationof the channel impedance 80 with the aperture reactance 76 and therecession impedance 72 is the parallel combination of the channelimpedance 80 and the aperture reactance 76 reflected to the beginning ofthe line representing the recession impedance 72. The total impedancepresented to the radiating element 14 at terminal pair 66 is thecombination of the resistor 68 in series with the parallel combinationof the aperture reactance 76 and the channel impedance 80 reflectedthrough the line representing the recession impedance 72. With anappropriate value for X a considerable variation in the fluid depth anda corresponding variation in the channel impedance 80 does notsignificantly alter the value of the total impedance appearing atterminal pair 66. In other words, the impedance presented to theradiating element 14 at terminal pair 66 is substantially constant withvariations in the depth of the fluid in the enclosed resonant drainagestructure. It is also evident that for values of X which aresubstantially smaller than the value S of the resistor 68, and forvalues of the recession in the ground plane which is small compared to awavelength, the total impedance presented to the radiating element 14 atterminal pair 66 is substantially the same as that shown in FIG. 4awhich depicts a situation analogous to a flush-mounted antenna array. Itis also evident that since the impedance is essentially constant withthe depth of the fluid in the enclosed drainage channel 46, theradiating elements 14 may be tuned for the impedance S +X p and that theradiating elements 14 will remain substantially tuned with variations inthe fluid depth. Also, the radiation characteristic of the antenna arrayis substantially the same as that of a flush-mounted array. It is thusevident that the invention of the enclosed resonant drainage structurewith apertures in the face of the array provides a ground plane and awide-band electrically resonant structure whose electricalcharacteristics approximate that of a flush-mounted array.

FIG. 5 illustrates one method of providing hemispherical coverage withmore than one antenna array 12. Here one array 12 is mounted on each ofthe four inclined roofs of the square-shaped building 82. The voidsbetween the lowest row of radiating elements 14 in each array 12 serveas exit ports 84 out of which the rainwater flows.

It should be understood that more than one form of connecting means isapplicable to bridge the drainage channels. While the present embodimentutilizes a connecting means in the form of the aforementioned shoes 22aflixed to the radiating elements 14, the connecting means couldalternatively have the form of metallic electrically conducting spacersadapted to make electrically conducting contacts with the radiatingelements at or adjacent to the front of the radiating elements, saidconnecting means being a part of the antenna ground plane. Theconnecting meas could alternatively be formed by a perforated metallicplate, screen or Wire mesh with openings of suflicient size to admit therainwater from the face of the array.

Other forms of connecting means will be apparent to those skilled in theart.

It should also be understood that more than one form of the radiatingelement 14 is applicable for the antenna array 12. For example,radiating element 14 or other type source of radiation can be terminatedin a window, lens or other suitable transparent termination. Radiatingelement 14 can alternatively be constructed in the form of a horn. Itmay contain a solid or fluid dielectric though air is usually employedfor the dielectric. If the dielectric were solid, no window would berequired insofar as the shedding of rainwater is concerned. However, awindow or lens might still be required for tuning the radiating element14.

FIG. 6 shows a phased array antenna system 86 including antenna array12. Details of such a typical system are described, for example, in U.S.Pat. No. 3,305,867, entitled Antenna Array System, issued to A. R.Miccioli et al., Feb. 21, 1967. In particular, a transmitting horn 88 inspaced-apart relationship from a microwave lens 90 generates a divergentbeam of microwave electromagnetic radiation whose rays 92 and wavefronts 94 are directed towards the microwave lens 90. The microwave lens90 is comprised of a plurality of horns 96, a corresponding set of phaseshifters 98, and a corresponding set of radiating elements 14 of theantenna array 12 raised or extending outward from support structure 16to shed water. Each phase shifter 98 directed by signals 100 from aprogrammer 102 imparts the requisite phase shift to the signaltransmitted by each radiating element 14 so that the antenna outputsignals 104 are collimated and steered in the requisite directions 106.

It is understood that the above-described embodiments of the inventionare illustrative only and modifications thereof will occur to thoseskilled in the art. For example, the concept of a tuned structureforming a part of a radiating element or an array of radiating elementsis not limited to electromagnetic radiation. The concept can be applied,for example, to acoustic radiators. Accordingly, it is desired that thisinvention is not to be limited to the embodiments disclosed herein butis to be limited only as defined by the appended claims.

I claim:

1. An antenna array comprising a plurality of radiating elements, anelectrically conductive support structure adapted to msition theradiating elements, said support structure being positioned behind theface of the array 9. suflicient distance to provide channels between theradiating elements permitting drainage of fluid from the face of thearray, and means for providing a ground plane at the face of said array,said means cooperating with the side surfaces of said radiating elementsand said support structure to permit resonant modes of radiant energywithin said channels.

2. Apparatus as defined in claim 1 wherein each of said radiatingelements is an open-ended cavity.

3. An antenna array comprising a pluralityof radiating elements, asupport structure adapted to position the radiating elements, saidsupport structure being positioned behind the face of the array asufficient distance to provide spaces between the radiating elementspermitting drainage of fluid from the face of the array, and means forproviding a ground plane for said array, said ground plane includingconnecting means for bridging the spaces between the radiating elements,said connecting means cooperating with said support structure to providesubstantially the same electrical characteristics as that of a groundplane located adjacent to the face of the array.

4. Apparatus as defined in claim 3 wherein said connecting means bridgesonly a portion of the space between the radiating elements therebyproviding apertures be tween individual elements to permit the drainageof fluid from the face of the array through said apertures.

5. Apparatus as defined in claim 4 wherein the support structure is anelectrically conducting plate located a specific distance behind theconnecting means to provide in conjunction with said radiating elementsa microwave cavity-type-structure.

6. Apparatus as defined in claim 5 wherein each of said radiatingelements is an open-ended cavity.

7. An antenna array comprising a plurality of radiating elements, eachof said radiating elements being an openended cavity, a supportstructure adapted to support and position the radiating elements, saidsupport structure being positioned behind the face of the array, meansfor providing a ground plane for said antenna array wherein said groundplane includes connecting means for bridging the spaces between theradiating elements, said connecting means cooperating with said supportstructure to provide substantially the same electrical characteristicsas that of a ground plane located adjacent to the face of the array;said support structure comprising an electrically conducting platelocated a specific distance behind the connecting means to provide inconjunction with said radiating elements a microwave cavity-typestructure; said connecting means bridging only a portion of the spacebetween the radiating elements thereby providing apertures between theindividual radiating elements to permit the drainage of fluid from theface of the array through said apertures into the microwavecavity-type-structure; said microwave cavity-type-structure beingadapted to serve as a drainage region to receive and drain fluid fromthe face of the array.

8. Apparatus as defined in claim 7 wherein the dimensions of theapertures and the dimensions of the microwave cavity-type-structure aresuch that the impedance characteristics of said apertures and structureand their effect on the radiation characteristics of the antenna arrayare essentially constant even with a changing depth of fluid in thedrainage region, the magnitude of the reactance of said apertures beingsufliciently small to permit the radiating elements to be tuned toprovide a radiation characteristic of the antenna array substantiallythe same as that of a similar antenna array wherein the radiatingelements are flush mounted.

9. Apparatus as defined in claim 8 wherein an exit port is provided topermit fluid to flow from the drainage region.

10. A phased array antenna including a plurality of cylindricalradiating elements and an electrically conductive support structureadapted to position the cylindrical radiating elements, said supportstructure being positioned behind the face of the array a suflicientdistance to provide channels between the cylindrical radiating elementspermitting drainage of fluid from the face of the array, said channelsextending in depth from the face of the array to said support structureand having a mean width which is sufficiently smaller than the depthsuch that the cross sectional form of the channels permits a uniformmode of radiant energy within the channels even with a changing depth offluid in the drainage region.

11. A phased array antenna including a plurality of radiating elements,a support structure adapted to position said radiating elements, saidsupport structure being positioned behind the face of the array asufficient distance to provide spaces between the radiating elementspermitting drainage of fluid from the face of the array, and connectingmeans for bridging the spaces between the radiating elements, saidconnecting means cooperating with said support structure to providesubstantially the same electrical characteristics as that of a groundplane located adjacent to the face of the array.

5 providing essentially constant radiation characteristics for saidantenna array even with a changing depth of fluid in the drainageregion.

References Cited UNITED STATES PATENTS 2,863,145 12/1958 Turner 343-8953,045,237 7/l962 Marston 343754 3,259,902 7/1966 Malech 343-754 ELILIEBERMAN, Primary Examiner US. Cl. X.R 343-846, 854

